Recently, highly-active supported transition metal compound catalyst combinations based upon the use of magnesium compounds have become available for the commercial production of polyethylene. For example, German Offenlegunsschrift No. 2,123,356 teaches polymerization of ethylene with an alkylaluminum compound promoter and a solid catalyst component made from a magnesium dialkoxide, a titanium (IV) compound and an alkylaluminum halide. Many of these catalysts, however, create serious problems in the particle-form and vapor phase ethylene polymerization processes because the particle size of the polymeric product is so fine that difficulties are encountered in the separation and handling of the polymer products; particularly, trouble occurs in the extrusion and compounding steps involved in producing a commercial product.
Now, new catalyst combinations have been found which have the high activities of the new generation supported and promoted transition metal catalysts but produce polymer of sufficiently large particle size so that the above-referred to problem is reduced or eliminated completely. The solution to this problem involves using a new solid catalyst component made using hydrogen chloride.